This invention generally relates to acoustic liners, and more particularly, to annularly or circumferentially shaped acoustic liners. Even more specifically, the present invention relates to a high efficiency broad band acoustic liner of the type especially well-suited to line the interior of a duct or shroud of a jet engine.
Acoustic liners are employed in many applications to attenuate noises generated by machinery or equipment; and, for instance, jet engines are almost universally provided with sound absorption liners or panels to attenuate sound waves produced inside the engines. One type of sound absorption liner commonly used in jet engines comprises a sound permeable facing sheet, a sound impermeable backing sheet and a honeycomb core interposed between these two sheets. Such devices are generally referred to as laminar absorbers, and one such absorber is disclosed in U.S. Pat. No. 3,166,149.
These prior art panels are simple, strong and light weight, and heretofore have generally produced acceptable results. Government regulations limiting the level or amount of noise that may be emitted from a jet engine are becoming stricter, though, and it may be very difficult for many common types of jet engines to comply with these more stringent noise limits using conventional prior art laminar sound absorbers. A principle reason for this is that most laminar absorbers are able to absorb sound effectively only at certain discrete frequencies, and between these discrete absorption bands, the absorption falls to a very low level.
Various attempts have been made to broaden the frequency range over which laminar absorption panels effectively attenuate sound waves; however, heretofore these attempts have not yielded any commercially practical designs. For example, a broader sound absorption characteristic may be obtained by providing the absorption panel with plural layers of permeable sheets and honeycomb cores, and examples of prior art devices of this general type are shown in U.S. Pat. Nos. 3,439,774; 3,640,357 and 3,670,843. These prior art broad band acoustic liners are bulky and heavy, though, and are difficult to manufacture in a commercially practical manner. Another approach to increasing the frequency range over which laminar absorption panels effectively attenuate noises involves modifying the shape and design of the honeycomb structure, and examples of this approach are found in U.S. Pat. Nos. 4,421,201; 3,913,702 and 3,831,710. These attempts usually result in a complex honeycomb design that also is difficult and expensive to manufacture.